4-Aminoquinolines as a novel class of NR1/2B subtype selective NMDA receptor antagonists

Article abstract of DOI:10.1016/S0960-894X(02)00470-5

Screening of the Roche compound library led to the identification of 4-aminoquinoline 4 as structurally novel NR1/2B subtype selective NMDA receptor antagonist. The SAR which was developed in this series resulted in the discovery of highly potent and in vivo active blockers.

Full text of DOI:10.1016/S0960-894X(02)00470-5

Bioorganic & Medicinal Chemistry Letters 12 (2002) 2615–2619
4-Aminoquinolines as a Novel Class of NR1/2B Subtype
Selective NMDA Receptor Antagonists
Emmanuel Pinard,a,* Alexander Alanine,a Anne Bourson,b
Bernd Buttelmann,a Marie-Paule Heitz,a Vincent Mutel,a
Ramanjit Gill,b Gerhard Trubeb and Rene Wylera
aPharma Division, Discovery Chemistry, F. Hoffmann-La Roche Ltd., CH-4070 Basel, Switzerland
bPharma Division, Preclinical CNS Research, F. Hoffmann-La Roche Ltd., CH-4070 Basel, Switzerland
Received 20 February 2002; revised 17 May 2002; accepted 17 June 2002
Abstract—Screening of the Roche compound library led to the identification of 4-aminoquinoline 4 as structurally novel NR1/2B
subtype selective NMDA receptor antagonist. The SAR which was developed in this series resulted in the discovery of highly potent
and in vivo active blockers. # 2002 Elsevier Science Ltd. All rights reserved.
Overactivation of NMDA receptors and the resulting
calcium overload of neurones is considered to be the
main contributor to neuronal cell death following acute
cerebral ischemia. NMDA receptor antagonists have,
thus, been shown to be potent neuroprotective agents in
animal models of focal cerebral ischemia.1,2
Native NMDA receptors exist as heteromeric assemblies
containing NR1 subunits together with one or more of
the four NR2 subunits (NR2A-D).3,4 During the last
decade, a number of NR1/2B subtype selective blockers5
have been described such as ifenprodil 1,6 CP-101,606
2,7 and Ro-25–6981 3.8 These compounds showed neu-
roprotective effect in vivo without inducing the side effects
associated with many non-selective NMDA receptor
antagonists.9À12 In addition, recent studies demonstrated
that ifenprodil 1 produces a state-dependent block of
NMDA receptors.13 This makes NR1/2B subtype selec-
tive blockers potentially attractive drugs for the treatment
of neurodegenerative diseases such as stroke,2 brain
trauma,2 pain12,14 and also Parkinson disease.15
include two aromatics, one central and usually basic
nitrogen and a phenolic hydroxyl group or a bioisosteric
phenol replacement.16
As part of our program, we set out to discover potent,
in vivo active NR1/2B suptype selective blockers
structurally unrelated to ifenprodil 1. Toward this end,
screening of the Roche compound library using tri-
tiated Ro-25–6981 as the radioligand in the binding
assay was performed. This screening campaign led to
the identification of the 4-aminoquinoline 4.
Strikingly, most NR1/2B subtype selective NMDA
receptor antagonists described so far are structurally
related to ifenprodil 1. Key elements of the proposed
pharmacophoric model in this series of compounds
*Corresponding author. Tel.: +41-61-688-4388; fax: +41-61-688-
8714; e-mail: emmanuel.pinard@roche.com
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00470-5

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E. Pinard et al. / Bioorg. Med. Chem. Lett. 12 (2002) 2615–2619
Scheme 1. (a) cat. TsOH, toluene, 110 ꢀC, Dean–Stark then Ph2O, 260 ꢀC, 32%; (b) POCl3, 80 ꢀC, 64%; (c) R1X (X=Br or I), nBuLi, THF, À20 ꢀC
to rt then H2O, I2, NaOH, rt, 30–80%; (d) 1,2,3,4-tetrahydroisoquinoline, toluene, 110 ꢀC, 84%; (e) RNH2, 150–200 ꢀC, 60–100%; (f) PDC, DMF,
rt, 6%.
Results and Discussion
Using 4 as a lead compound, potent and in vivo active
NR1/2B suptype selective NMDA receptor antagonists
were designed.
Our first aim was to define the essential structural ele-
ments required for in vitro activity.
As shown in Table 1, starting from the screening hit 4,
activity was abolished by removing the aminopro-
pandiol moiety (5) and recovered after introduction of
the amino group at position 4 of the quinoline ring (6).
This result demonstrated that the nitrogen atom at
position 4 is crucial for activity. As expected, introduc-
tion of the nitrogen atom has a profound effect on the
basicity of the quinoline ring (see Table 1). It renders it
sufficiently basic so that it is protonated at physiological
pH (pK>8.4). This observation strongly suggests that the
protonated quinoline is most likely to be the bioactive
form which interacts with the NMDA receptor.
Chemistry
The main synthetic scheme for the preparation of
4-aminoquinolines is straighforward using established
synthetic methods and is outlined in Scheme 1.
The 4-amino groups of compounds 4, 6–27, 29–32 were
introduced by heating the corresponding 4-chlor-
oquinolines 33–47 with an excess of primary amines in
the absence of any solvent. Ketone 28 was produced by
oxidation of aminopropan-2-ol derivative 22 (see Table
3) in the presence of PDC. Depending on the nature of
the substituents located at position 2 of the quinoline
ring, the non commercially available 4-chloroquinolines
33–44 were prepared according to three different routes.
Following route one, 4-chloroquinoline 43 was pro-
duced by chlorination of the corresponding quinolone
4817 which was obtained by thermal condensation of
aniline and b-ketoester 49.18 Following route two,
4-chloroquinolines 33–42 were obtained by adding alkyl
or aryl lithium derivatives to 4-chloroquinoline followed
by aromatisation of the intermediates 1,2-dihydro-
quinolines with iodine and base.19,20 Finally, according
to route three, 4-chloroquinoline 44 was prepared in a
regioselective manner by condensation of 2,4-dichloro-
quinoline with 1,2,3,4-tetrahydroisoquinoline.
In addition, it was found that the phenyl group at posi-
tion 2 of the quinoline system was also essential since its
deletion or its replacement by aliphatic groups resulted
in virtually inactive compounds (7–9).
Having identified these two important structural ele-
ments, the next step was to explore chemical variations
of the 2-aryl ring and of the 4-amino group.
As shown in Table 2, introduction of a substituent in
ortho position of the 2-aryl ring resulted in the weakly
active compound 10, suggesting that coplanarity of the
aromatic ring with the quinoline nucleus is required.

E. Pinard et al. / Bioorg. Med. Chem. Lett. 12 (2002) 2615–2619
Table 1. NMDA affinities of compounds 4–9: influence of pKa
2617
substituents (15) at the para position led to a significant
drop of affinity. This result strongly suggests that the
aromatic moiety fits in a size limited hydrophobic
pocket of the NMDA receptor. Compounds substituted
at both the meta and para position by small and lipo-
philic substituents generally showed high activity as
exemplified with compounds 16 and 17. Low nanomolar
activity was also obtained for the annelated derivative
18. Interestingly, the regioisomeric tetrahydro-
naphtalene derivative 19 was found to be fairly active
(100 nM) demonstrating that the aromatic group does
not need to be directly attached at position 2 of the quin-
oline ring. Gratifyingly, a 5-fold increase of activity was
observed by attaching the quinoline ring by the means of a
C–N bond instead of a C–C bond as illustrated by the
tetrahydroisoquinoline derivative 20 (11 nM).
c
Compd
R1
R2
Ki (nM)a NMDAb
pKa
4
260
8.4
5
6
7
8
9
H
>35,000
413
5.2
8.5
NH2
H
>35,000
>40,000
33,000
n.d.
n.d.
n.d.
By keeping the phenyl group fixed at C2, variation of the
C4 position was explored on the quinoline ring system.
CH3
As exemplified in Table 3, starting from the lead com-
pound 4, deletion of both OH groups resulted in nearly
20-fold decrease of activity (21) demonstrating that at
least one OH group is mandatory for good binding
affinity. The fact that activity was retained with the
racemic 2-amino propanol derivative 22 and dramati-
cally reduced with the aminopropanol derivative 23
demonstrated that the important OH group is the one
located at the vicinal position to the nitrogen atom. Of
the two possible enantiomers of 22, the R-configurated
2-aminopropanol derivative 24 displayed the best affi-
nity for the NMDA receptor (75 nM). The same range
of activity was also attained with the non-chiral etha-
nolamino derivative 26. As illustrated by compounds 27
(CH2)3CH3
aBinding affinities are quoted as Ki values and are the geometric mean
of at least two experiments. The variability is less than 20%.
bDisplacement of [3H]-25–6981.21
cpKa values were determined using a potentiometric method.22
Furthermore, introduction of small and lipophilic sub-
stituents in the para position (11–13) showed a bene-
ficial effect. In particular, highest activity was obtained
in the case of the presence of electron donating sub-
stituents. Introduction of larger (14) or more polar
Table 2. NMDA affinities of compounds 10–20; variation of R1
Compd
R1
Ki (nM)a NMDAb
Compd
R1
Ki (nM)a NMDAb
10
3700
16
17
11
12
13
14
15
23
19
17
18
19
20
26
22
100
100
11
500
>4000
aSee Table 1.
bSee Table 1.

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E. Pinard et al. / Bioorg. Med. Chem. Lett. 12 (2002) 2615–2619
Table 3. NMDA affinities of compounds 21–28; variation of R2
Comp
R2
Ki (nM)a NMDAb
Compd
R2
Ki (nM)a NMDAb
21
4900
25
450
22
23
24
260
1900
75
26
27
28
110
7500
7900
aSee Table 1.
bSee Table 1.
Table 4. NMDA affinities and in vivo activities of compounds 29–32
Compd
R1
R2
Ki (nM)a NMDAb
ED50 (mg/kg)c
Sound induced seizures
29
3.5
<12
12
30
31
32
10
10
15
17
25
aSee Table 1.
bSee Table 1.
cCompounds were administered ip 30 min before testing in DBA/2 mice.
and 28, the potency was severely reduced after methy-
lation or oxidation of the vicinal OH group, suggesting
that this group acts as an hydrogen bond donor.
derivatives exhibited good in vivo activity indicating
that they penetrate well into the brain.
The next step in our optimization phase was to combine
the best substituents which were previously identified at
position 2 and 4. As exemplified in Table 4, the effect of
substituents was found to be additive and compounds
with much improved in vitro activity were identified. In
particular, exceptionally high potency (3.5 nM) was
achieved with the tetrahydroisoquinoline derivative 29
containing the ethanolamino group at the C4 position.
Conclusion
Starting from 4-aminoquinoline 4 as a screening hit, a
SAR was developed which resulted in the discovery of
highly in vitro and in vivo potent NR1/2B subtype
selective NMDA antagonists structurally unrelated to
ifenprodil. Within this novel series, the pharmacophore
elements identified were: (1) a protonated quinoline
ring, (2) a lipophilic aromatic system attached at the C2
position and (3) a hydrogen bond donating group
linked vicinaly to a C4-nitrogen on the quinoline fra-
mework. Since the nature and the spatial arrangement
In vivo activity was measured in mice after ip adminis-
tration23 using the standard sound-induced seizures
assay.24 As depicted in Table 4, the 4-aminoquinoline

E. Pinard et al. / Bioorg. Med. Chem. Lett. 12 (2002) 2615–2619
2619
of these pharmacophore elements differ notably from
those identified in the ifenprodil series, it is suggested
that the 4-aminoquinolines reported here and the ifen-
prodil like compounds might not fully share the same
binding site at the NMDA receptor.
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